To be or not to be controlled? The ecological role of the Guiana dolphin in its southernmost range

Several studies have explored dolphins feeding ecology in coastal ecosystems; However, research gauging the predatory effects of delphinid on ecosystems, which can be applied to management and conservation efforts, is still lacking. The northwestern section of the bay of Santa Catarina Island, locally known as North Bay, in Southern Brazil, inhabits the southernmost population of the Guiana dolphin (Sotalia guianensis). This small and resident population is structured in a single and highly cohesive group, using a small home range restricted to the western section of the bay. Like many other coastal dolphins, this dolphin population negatively interacts with fisheries, with frequent bycatch events or by sharing feeding resources. To explore the ecological role of dolphins in the North Bay ecosystem, we constructed an Ecopath food web model to estimate ecosystem and compartmental attributes. Mixed trophic impact and keystoneness index were used to assess the predatory effects of the species in the ecosystem. As a complementary approach to Ecopath, we used ecological network analysis (ENA) to determine the ecological role of the compartments using control analysis and the throughflow centrality. According to our model, North Bay is a mature detritus-based ecosystem. Dolphins showed the highest keystoneness index score among all functional groups, while control analysis showed they are also the most controlled component in the ecosystem. The dolphins rely mainly on the Whitemouth croaker (Micropogonias furnieri), a hub species showing the greatest centrality of flows. We concluded that the Guiana dolphin inhabiting the North Bay is a crucial compartment in the ecosystem. It shows the greatest ecosystem impact per biomass unit among all functional groups, while it is the most vulnerable compartment.